Design, Assembly and Testing of a Mobile Laboratory Based on a VTOL Scale Motorglider

First step of the process is to design a proper layout capable of connecting all motors to the fixed wing structure. The idea is to realize two different frames, one for each wing, where all four motors could be mounted. In particular each frame should sustain two motors, a rear fixed and a front tiltable ones (Figure 1)

A scale glider model named “Pilatus” has been chosen to being modified, due to its strength, easy maneuvers skills, excellent flight and acrobatic performances.  Table 1 shows its main characteristics.

1.png

Figure 1. VTOL configuration

Table 1. Pilatus characteristics

2.1 Motor’s frame design

In the adopted solution, each frame is composed by two carbon fiber plates connected together by aluminum blocks to satisfy light weight and strength requirements. Each plate is realized from 3 mm thick carbonfibre material shaped at water high pressure cutting machine.

Each structure provides two connecting spots for the motors: a fixed rear one named (A) where motors plate is mounted forcing lift along vertical direction and a front rotating one, named (B), where propeller-motor coupling is able to tilt according to desired flight configuration (Figure 2).

2.png

Figure 2. Motor frame design

A profiled housing has been realized on each carbon fiber frame to couple the wing airfoil outline so to provide a strength and precise connection frame-wing (Figure 3).

3.png

Figure 3. Frame/Wing Coupling system

To avoid undesired wing twist during take-off and landing procedure is crucial that thrust center coincides with center of gravity. Furthermore is important to maintain the structure as light as possible and ensure any interference between motor air flow and wing area. For these reasons, a proper frame dimensions have been chosen (Table 2).

To distribute the pressure force along wing/frame connection and have the possibility to regulate the angle of attack of all system, we decide to design a 140 mm profiled area to couple the HQ2/12. A detailed imagine about the fixing system which let us easily fit and remove the motor frame is shown in Figure 4.

Table 2. Frame dimensions

4.png

Figure 4. locking system

It consists of two separated components (A & B) connected by solid aluminum blocks. This system allows a rotational movement around a fixed-point C and provides a closing point D in such a way to insert and fix the wing within the carbon frame. As first approach we decide to take the horizontal frame line parallel to the wing chord in a way to obtain a glider leveled hovering flight.

For better understanding forces and displacement behavior, a static load FEM analysis has been carried out using a cad simulator. In particular has been used a linear elastic orthotropic model embedded within the program using Carboresine as material assigned. Carbon characteristics used are listed in Table 3.

Table 3. Carbon fiber characteristics

Table 4. Calculated masses

Firstly, to simulate frame performance, a complete VTOL plane weight of 6500 gf has been supposed considering a Load Capacity ‘LC’ limit of 120 g/dm² with a wing span of 56.6 dm². Actually a total real weight of 6350 gf will be measured later.

To evaluate the right pressure which has to be applied along to the connecting area, we had to reduce the estimated total weight force by the engine and propeller masses which are self-sustaining. In Table 4 masses considered are listed.

Each frame is composed by two carbon fiber plates, the calculated pressure (P) applied over all the Contact Area “A_c” (Figure 5) should be:

$M_{\text {plate }}=\frac{M_{\text {noprop }}}{4}=1150[\mathrm{gf}]$         (1)

$F_{\text {plate }}=M_{\text {plate }} \cdot g \cdot X=22563[N]$            (2)

$P=\frac{F_{\text {plate }}}{A_{c}}=53593\left[\mathrm{~N} / \mathrm{m}^{2}\right]$          (3)

5.png

Figure 5. FEM simulation Pressure applied

Fixed geometry constrains with cylindrical radial distribution have been applied on all 13 holes where spacers and motor plates connection screws are situated (Figure 6).

6.png

Figure 6. Static constrains

The simulation of static load analysis has returned results related to the calculation of stress, displacement and deformation of the plate under the imposed loads. A brief summary is presented below.

7.png

Figure 7. Stress calculation – Von Mises criteria

Compared to maximum tensile strength of σmax = 4.480∙10⁹ [N/m²], we can say that it works under large safety margins.

8.png

Figure 8. Displacement criteria

9.png

Figure 9. Deformation criteria

2.2 Motor testing and tilt system design

Considering a load capacity limit of 120 g/dm², an overall weight of 6500 gf has to be considered to determinate the motors suitable for this application.

Trough eCalc RC calculator, an open source selecting program, an outrunner brushless electric motor powered by 24V direct current with permanents magnets has been chosen.

The thrust force of each motor is 2320 gf powered with 22 Volt with an absorbed current of 17 A. Technical data are reported in Figure 10.

10.png

Figure 10. Brushless motor data

To verify reported data and evaluate the real electrical absorption of engine/prop combination, a motor test bench has been realized (Figure 11). Motor has been powered using a 6S LiPo battery which has a full charge voltage of 25.2 V. Table 5 shows the thrust and electrical absorption from 0 to 100% throttle.

11.png

Figure 11. Motor test bench

Table 5. Test bench results

Several test sessions have been done, resulting in a proportional behavior compared to the battery state of charge. It is important to always flight with a full charged battery. With a 6S battery it is possible to have a stable hovering flight with a 65% of throttle. The knowledge of this parameters will be crucial during transition programming stage.

Front motors tilt system is carried out by a mechanical parallelogram system which transfers the rotary servocontrol movement to a motor support as shown in Figure 12. Engine and props may rotate from horizontal to vertical position by controlling a servo motor capable of 90° rotation.

12.png

Figure 12. Motor tilt system

Lightness, simplicity and durability have been the main focus during tilt system design. This solution let us use a Savox Servo model SA-1256TG which weights 64 gf, has a speed of 0.15 sec/60° and has a torque force of 20 Kgcm powered with a direct current of 6 V. It’s a full digital servo motor which allows us to fine control all the parameters during transition such as the progressive inclination of the front motors and the rotation speed during take-off/forward flight switch and vice-versa.

2.3 Avionics components

To perform different flight configurations, a controller should be mounted to ensure easy and stable transition between them. A “Navio 2” flight controller, manufactured by Emlid spa, is being chosen due to its excellent performance in fixed wing controlling and his compatibility with Raspberry Pi 3 B+ mini-PC. This is a reliable system where “Ardupilot”, an open-source autopilot program, can easily be installed and used to control all different flight phases (Figure 13).

13.png

Figure 13. “Navio 2” flight control

Table 6 shows other electronics components adopted.

Table 6. Electronics components

To understand VTOL behavior, two different layouts must be studied. Vertical take-off in multi rotors quad mode configuration and the transition to forward flight as fixed wing setup.

4.1 Vertical take-off and landing flight phases

At first phase, our model should takeoff vertically as a multirotor UAV using motor’s lift. Unlike traditional UAV’s, unbalanced forces are generated due to asymmetrical geometry and aerodynamics areas which works as drag flat surfaces. Balance around roll axis has been investigated as bidimensional configuration considering weight and drag forces respectively generated by fuselage, main and tail wings [15] (Figure 19).

19.png

Figure 19. Vertical take-off forces

Drag and moment forces are related to vertical speed. From Figure 20 a suitable value is 1.5 [m/s].

It can be notice that tail drag force gives the greatest contribution to unbalance the aircraft during vertical take-off. Therefore, rear motors must create a higher lift than front motors to stabilize the maneuver. At this vertical speed, motor glider will take 45 sec to reach an altitude of 40 m which is the safest minimum quote requested to start transition sequence.

20.png

Figure 20. Drag and Moment behaviour

4.2 Transition from multirotor take-off to fixed-wing flying state

Leaning on the electronic stability control provided by the flight controller, the transition sequence starts once a stable hovering flight is reached. During this stage, the relative wind generated by the aircraft motion substantially changes the vehicle dynamic behavior between hovering and cruise flight modes. In helicopter-flight mode (VTOL and hovering), the deflections of aerodynamic surfaces (aileron, rudder, and elevator) do not produce significant dynamical effects, whereas in cruise-flight mode, small deflections produce significant aerodynamic forces. They can be used to generate both the necessary aerodynamic forces to sustain forward flight, and the aerodynamic moments that allow control and guidance. The aircraft stability is demanded to motors during take-off and landing phases, whereas aerodynamics surfaces have the complete attitude control during forward flight time.

Transition between hovering to forward flight basically consist of 3 distinct steps:

STEP 1

As a stable hovering flight is reached, front motors begin to tilt to generate horizontal thrust. Front motors should perform a higher thrust than the rear motor to achieve a vertical component “T₁₂” equals to “T₃₄” (Figure 21). This ensures a minimum altitude loss and good stability but leads to a higher electric consumption. At this stage all four motors are running under “Navio” controlling.

STEP 2

The higher true air speed is reached, the more increased motor rotation is achieved. The driven digital servo allows us to set up a different degree of rotation compared to measured velocity. In this phase aerodynamics surface begin to generate lift and less motor thrust is demanded to maintain altitude and stability.

STEP3

One a true air velocity of 17.25 m/s “Vref” is reached, front motors move to horizontal position to guarantee forward thrust while rear motors stop spinning. In this phase glider weight is totally hold up by the aerodynamics forces. Fixed wing flight mode is activated and the complete aircraft control is left to the pilot who may use throttle to activate the front motors and gain altitude.

21.png

Figure 21. Transition forces

Subsequently to perform a stable vertical landing, a second transition between forward flight to hovering must be considered. At this stage a complete motor rotation from horizontal to vertical position is triggered by a remote control operated by the pilot.

It’s a challenging process where flight controller manages altitude loss, axis rotations and speed reduction until a stable hovering flight is reached. Front motors fully rotate to achieve vertical position and once transition has been completed, all controls are left to the pilot who may perform a vertical landing in multi copter flight mode.